The present invention relates to a fibrous material and its use as a filtration medium.
Filtration is the process of separating particles from a fluid suspension (liquid or gas) by passing the fluid through a filter medium (e.g., a septum, membrane, or porous medium). The particles may be coarse or fine. They may even consist of atoms or molecules. The particles may be inorganic or organic in composition and may be natural or biological in origin (examples of the latter include bacteria and viruses).
Such liquid-solid separations are important in the manufacture of chemicals, polymer products, medicinals, drugs and other pharmaceuticals, beverages, and foods; mineral and other metallurgical processing; petroleum refining and processing; water purification and filtration; sewage disposal; the chemical laboratory; specific chemical processes (e.g., the dry-cleaning process); and the operation of machines such as internal combustion engines and refrigeration units.
In a typical filtration process, a filter medium retains most of the solids on or within itself, but allows the fluid being filtered to pass through it unimpeded. A driving force, usually in the form of a static pressure difference across the filter (generated by gravity, vacuum applied downstream, external pressure applied upstream, or by centrifugal force), must be applied to achieve flow through the filter medium (a general review of filtration is contained in C. Dickenson, Editor, "Filters and Filtration Handbook," 2nd Edition, The Trade & Technical Press, Ltd., Morden, Surrey, 1987). For a review of particle removal from liquids, see Chapter 8 in T. H. Meltzer, "Filtration in the Pharmaceutical Industry," Marcel Dekker, New York, 1987.
Filtration often is referred to as a mechanical separation because the separation is accomplished by physical means. This does not preclude the use of chemical or thermal pretreatment to enhance filtration. Cationic flocculants have been used to enhance filtration, but they also can clog the filter medium, especially if used in excess. Similarly, polyelectrolytes have been used extensively in the treatment of water and wastewater, but as flocculants or coagulants prior to sedimentation and filtration (see, for example, W. L. K. Schwoyer, Editor, "Polyelectrolytes for Water and Wastewater Treatment," CRC Press, Boca Raton, 1981).
Particles are collected by filters via the following mechanisms: direct interception, inertial impaction, and diffusion. Collection takes place on the solid elements of the filter. Direct interception occurs when a particle collides head-on with a solid element at the surface of the filter. Inertial impaction results if a particle in the stream fails to negotiate the tortuous path through the filter bed and adheres to a solid element inside the filter. Diffusion occurs when extremely small particles (i.e., particles having diameters less than 1 micrometer) wander via Brownian motion within the flow of the fluid stream, thereby augmenting their chances of intersecting the solid elements forming the filter medium.
The two broad categories of conventional filters are surface-type filters and depth-type filters. The feed to surface filters normally contains at least 1 percent by volume solids, while deep-bed filtration is applied to very dilute suspensions of solids concentrations, e.g., less than 0.1 percent by volume. Surface and depth filtration also have been used together, as in the cartridge filtration of extreme fines, such as bacteria.
These two types of filters function in very different ways. First, surface filtration works via direct interception. Particles larger than the pore size of the medium are stopped at the upstream surface of the filter; their size prevents them from entering and/or passing through the pores. They are, in effect, strained out of the fluid stream. Adsorptive forces, while present, are small in magnitude and unimportant to this filtration process. When surface-type filters are exposed to the flow of a contaminated fluid, the filter quickly clogs due to two processes:
i) The effective pore-size of the medium is reduced, as some of the pores become partially blocked by particles. This pore blocking can be caused by adsorption of small particles within the pores.
ii) A `cake` of successive layers of solids deposits and builds up on the surface of the medium; the cake itself forms a filter which clogs with time. The selectivity of the filter is thus determined by the contaminant particles and not by the original filter.
Surface filters are of three broad types: screens, edge-type filters (where the flow takes place from the edge inwards), and stacked disc filters (where the flow is between and through filter discs and then into an inner tube).
Second, depth filtration employs a medium of a sufficient thickness so as to filter in its bulk. The filter medium typically is a deep bed with pore sizes much greater than the particles it is meant to remove, so the particles penetrate into the medium. The random path through the depth filter is much longer than that through a surface filter, providing a greater possibility for retention. Retention efficiency in conventional deep-bed filters is achieved by means of a series of low-efficiency particle captures. Adsorptive surface forces (molecular and electrostatic) can enhance attachment to the medium, which then improves retention within the filter.
Depth-type filters generally are made from fibrous, porous, or caked media. Fibrous media constitute a layer, or mat, of numerous fine fibers (e.g., fiber diameters ranging from 0.5 to 30 micrometers). These fibers are randomly oriented, thereby creating the numerous tortuous passages or pores in which the particles are trapped and held by the previously described mechanisms. Commonly used fibrous materials are cellulose, cotton, glass, and synthetics (e.g. rayon, polypropylene).
Either type of filtration may utilize affinity filtration. Affinity filtration, which separates chemically distinct particles, makes special use of electrostatic, hydrophobic, or specific chemical interactions between the particles and the filter medium. Attachment forces often are increased by small amounts of previously deposited particles.
Notwithstanding past improvements in filter media, there still is an opportunity for an improved filter medium, particularly a filter medium adapted for use in depth-filtration processes.